The present invention relates generally to a micro-electromechanical apparatus and method, and more particularly to a micro-electromechanical apparatus and method with position sensor compensation.
In recent years optical systems have come into widespread use in a wide variety of applications. In wired systems, optical signals are transmitted along optical fibers and switched from one fiber to another with some type of switch. In one prior art switching method, an optical signal received on an optical fiber may be converted into an electrical signal, switched in the electrical domain, and then converted back into an optical signal for transmission on a different optical fiber.
In order to avoid these optical domain-electrical domain conversions, another prior art switching method mechanically positions the end of an optical fiber in one fiber group so as to point toward the end of another optical fiber in another fiber group. While this method keeps the signal in the optical domain, it generally requires a fairly complex and relatively slow positioning and alignment mechanism that is duplicated for each fiber in a group. There is thus a need for an optical steering device that keeps the signal in the optical domain, but does not have the problems associated with the fiber-positioning apparatus.
In another optical application, a wireless network may be implemented that does not use a physical wire for transmission of the light signal. Data is transmitted through the modulation of a light beam, in much the same manner as wired fiber optic communications. A photoreceiver receives the modulated light and demodulates the signal to retrieve the data. In the case of directed optical communications, a line-of-sight relationship between the transmitter and the receiver permits the light beam to travel without a fiber optic waveguide. Such a wireless system may be used for inter-device communication within a small room, for a local network within a building or between buildings, or for an external network.
The proper functioning of a wireless system generally relies on the ability to correctly aim the transmitted light beam to the receiver. For example, a laser-generated collimated beam can have a small spot size, and the reliability and signal-to-noise ratio of the transmitted signal are degraded if the aim of the transmitting beam strays from the optimum point at the receiver. There is thus a need for an optical steering device that can precisely position a light beam on a target receiver, and keep it on the receiver under changing conditions.
Laor et al., U.S. Pat. No. 6,295,154, issued Sep. 25, 2001, entitled OPTICAL SWITCHING APPARATUS,xe2x80x9d commonly assigned herewith and incorporated herein by reference, discloses a micro-electromechanical (xe2x80x9cMEMxe2x80x9d) movable mirror assembly that may be used to alleviate the above problems in wired and wireless optical applications. As described in detail in Laor et al., with respect to a wired system, a micromirror is positioned relative to optical fibers so that it may direct optical signals from a fiber to one or more different optical fibers. This apparatus may also be used to direct and align a wireless optical signal between a transmitter and a receiver.
The micromirror generally is rotatable about two axis and is driven magnetically using some combination of permanent magnets and electromagnetic coils. The micromirror reflects the light signal in a manner that may be precisely controlled by the electrical signals sent to the electromagnetic coils. Because analog signals are used to control the coils, the mirror""s position is generally continuously variable over its range of motion. The precise positioning of the micromirror is generally accomplished by way of calibration and feedback, so that the optical system is able to sense the mirror""s position and make corrections.
Copending application Ser. No. 09/957,476, filed Sep. 20, 2001, entitled PACKAGED MICROMIRROR ASSEMBLY WITH IN-PACKAGE MIRROR POSITION FEEDBACK, discloses a micromirror apparatus with micromirror position sensing implemented into the apparatus. As disclosed in detail in the copending application, underlying the mirror is a sensor for sensing the angular position of the mirror. In one embodiment, the sensor includes a light-emitting diode (xe2x80x9cLEDxe2x80x9d) and angularly spaced light sensors (preferably four, one in each quadrant) that can sense the intensity of light emitted by the diode and reflected from the backside of the mirror. The position of the mirror may be derived from a comparison of the intensities sensed by the various angularly positioned light sensors. This calculation is generally performed by a digital signal processor (xe2x80x9cDSPxe2x80x9d) or general purpose microprocessor which is controlling the mirror""s position.
One potential problem with using an LED for position feedback is that the output from the photodetectors is a function not only of the position of the mirror, but also of the LED intensity and detector sensitivity, with are both generally a function of temperature and, to a lesser extent, time. Generally, this will introduce drift into the feedback signals (the voltage outputs from the photodetectors), causing the indicated position of the mirror to drift as well. One method of compensating for this drift is to divide each position calculation by the sum of the voltages output by the detectors. A disadvantage of this method, however, is that the processor is performing mirror positioning in real time, and division is a time-consuming operation, generally taking many clock cycles to complete. Thus, the processor may not be able to compensate for LED intensity and detector sensitivity drift, and still control micromirror positioning in real time.
These and other problems are generally solved or circumvented, and technical advantages are generally achieved, by preferred embodiments of the present invention in which position sensor compensation is performed without using valuable processor resources to perform a time-consuming division operation. Generally, in preferred embodiments, instead of using division in the position calculation to compensate for changes in LED intensity and detector sensitivity, the intensity of the LED is modified by the controller to maintain a constant sum of the voltages output from the photodetectors.
Preferred embodiments of the invention may be implemented in either software in the processor or in an analog circuit tied into the feedback loop. In a preferred embodiment, the sum of the four sensor voltages is monitored and compared to a desired set point. If the sum is below the set point, the LED intensity is increased, and if the sum is above the set point, the LED intensity is decreased. The sum of the four sensor outputs is thus maintained at a constant voltage (the set point value) over time.
In accordance with a preferred embodiment of the present invention, a micro-electromechanical apparatus comprises an actuator element, a light source for illuminating a portion of the actuator element, a plurality of detectors for detecting a light intensity from the light source after reflection from the portion of the actuator element, wherein the light intensity detected at the plurality of detectors is representative of an orientation of the actuator element, and control circuitry for sending an intensity level signal to the light source and for receiving voltage signals from the plurality of detectors proportional to the intensity of the detected light. The control circuitry comprises a summer for adding the voltage signals to generate a voltage sum, and a comparator for comparing the voltage sum to an light intensity setpoint. The control circuitry adjusts the intensity level signal based on the voltage sum to light intensity setpoint comparison.
In accordance with another preferred embodiment of the present invention, a method of compensating for position sensor drift in a micromirror device comprises providing a light intensity level signal to a light source, directing light proportional to the light intensity level signal at an underside of a micromirror, detecting light reflected from the underside of the micromirror with a plurality of photodetectors, generating voltage signals representative of the reflected light detected with the plurality of photodetectors, summing the voltage signals to generate a voltage sum, comparing the voltage sum to a light intensity setpoint, and adjusting the light intensity level signal based on the voltage sum to light intensity setpoint comparison.
In accordance with another preferred embodiment of the present invention, a micromirror apparatus comprises a mirror element, a light source for illuminating a portion of an underside of the mirror element, a plurality of detectors outputting voltage signals representative of a light intensity detected from the light source after reflection from the underside of the mirror element, wherein the voltage signals are representative of an orientation of the mirror element, and sensor control circuitry for sending a light intensity signal to the light source and for receiving the voltage signals from the plurality of detectors. The control circuitry comprises a summer for adding the voltage signals to generate a voltage sum, and a comparator for comparing the voltage sum to an light intensity setpoint. The control circuitry adjusts the light intensity signal based on the voltage sum to light intensity setpoint comparison.
An advantage of a preferred embodiment of the present invention is that changes in LED intensity and detector sensitivity may be compensated for in an analog micromirror apparatus, providing more accurate micromirror positioning.
Another advantage of a preferred embodiment of the present invention is that position sensor compensation may be performed without a division operation.
Another advantage of a preferred embodiment, in which the compensation calculation is performed by the controlling processor, is that the compensation is not part of the position calculation, and therefore the calculations may be performed in non-critical time periods apart from the direct position measurement/control operation.
Another advantage of a preferred embodiment of the present invention is that when the compensation is performed by analog circuitry, the controlling processor generally is not required to perform any compensation calculations.
A further advantage of a preferred embodiment of the present invention, in which the compensation is performed by analog circuitry, is that the set point may be adjusted once, and then generally does not need to be updated. In addition, an analog implementation may allow a coarser, less expensive digital-to-analog converter (xe2x80x9cDACxe2x80x9d) to be used for setting the set point, because the analog circuitry will provide any fine adjustments to the set point.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures or processes for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.